1. Field of the Invention
The present invention relates to a method for manufacturing an insulated gate bipolar transistor (IGBT) of a light-punch-through type, and more specifically, to a method for manufacturing a semiconductor device that can prevent the lowering of destruction resistance in short-circuit test, and can elevate the switching speed while suppressing the fluctuation of ON voltages, and a method for manufacturing such a semiconductor device.
2. Background Art
An insulated gate bipolar transistor (IGBT) is widely used in power converters, such as inverters. The total loss when in IGBT is operated can be expressed by the total of steady loss determined by ON voltage and switching losses determined by the turn-on speed and the turn-off speed. To obtain a compact IGBT of a high cost performance, the total loss must be reduced. Therefore, efforts for lowering the total loss have been carried out by miniaturizing a surface MOSFET, converting a planar structure to a trench structure, or optimizing carrier distribution in the vertical direction.
IGBTs are driven at various frequencies depending on the uses, and it is also important for lowering total loss that the ON voltages and switching speeds are controlled to suite driving frequencies. For example, when an IGBT is driven at a high frequency, the proportion of steady loss in the total loss is lowered, and the proportion of switching loss is elevated. In this case, in order to lower the total loss, it is required to design an element having a high switching speed even if the ON voltage is somewhat high.
In IGBTs of the same MOS structure or cross-sectional structure, ON voltages and switching speeds are in a trade-off relation. Specifically, when ON voltage is lowered, the switching speed is also lowered, and when ON voltage is elevated, the switching speed is also elevated. The methods for controlling these are roughly classified into two methods. One is a method wherein the life time in the IGBT is controlled by radiating electron beams, protons, or helium ions (for example, refer to Japanese Patent Application Laid-Open No. 9-121052). The other is a method wherein the concentrations in the P-collector layer and the N-buffer layer on the back face are controlled to change the injecting efficiency of holes injected from the P-collector layer into the N−-drift layer in the ON state.
IGBTs can be classified into a punch-through type using an epitaxial wafer as the material, a non-punch-through type using a floating-zone wafer as the material, and a light-punch-through type that is an intermediate structure between them. Of these structures, recently, the light-punch-through type IGBTs, which have low material costs and excellent performances, have been actively developed.
FIG. 15 is a sectional view showing a conventional light-punch-through type IGBT. On the upper surface of a wafer 11, a plurality of MOSFETs each having an N-channel are formed in a stripe. Specifically, a P-base layer 13 is formed on an N−-drift layer 12, and an N+-emitter layers 14 are formed on the surface of parts of the P-base layer 13. Trenches are formed so as to penetrate the N+-emitter layers 14, and gate electrodes 16 are formed in the trenches via gate insulating films 15.
On the gate electrodes 16, insulating films 17 are formed. On the lower surface of the wafer 11, an N-buffer layer 21 is formed. On the lower surface side of the wafer 11, lower than the N-buffer layer 21, a P-collector layer 22 is formed. An emitter electrode 23 is formed on the upper surface of the wafer 11, and a collector electrode 24 is formed on the lower surface of the wafer 11.
The ON voltages and the switching speeds of the light-punch-through type IGBT is generally controlled by controlling the concentration in the P-collector layer. Specifically, the switching speed can be elevated by lowering the concentration in the P-collector layer to decrease the quantity of holes injected from the P-collector layer into the N−-drift layer. However, if the injection efficiency of holes is extremely low, field intensity in the P-collector side is elevated when the IGBT is in a short-circuit state. Since the parasitic thyristor in the IGBT is easily turned on by impact ions generated by the field intensity, a problem of lowered destruction resistance is caused.
In addition, if the concentration in the P-collector layer is lowered, the fluctuation of the contact resistance between the P-collector layer and the collector electrode formed by contact to the P-collector layer increases. Therefore, the problem of the large fluctuation of the ON voltage is caused.